This invention relates to a control circuit for mixing two or more signals and more particularly to such a control circuit when embodied in an RF equalizer.
It is desirable in the reproduction of recorded signals, in particular signals recorded on magnetic media, to compensate for frequency response roll-off. In video tape recording or reproducing this must be corrected before the signal enters the frequency demodulator. Even in an audio system it is desirable and sometimes necessary to correct for the amplitude roll-off with frequency.
In the past this has been corrected by an equalizer circuit which utilizes a time delay having a plurality of time delay elements connected in series. The end output of the time delay is combined together with taps along the series connected time delay elements in a known manner such that phase shifts produced by the time delays produce a combined signal having a relatively flat frequency response over a defined frequency range.
In situations where the reproduction of the recorded signals involve the use of a plurality of magnetic recording heads, such as the rotating magnetic heads in a video tape recording and reproducing apparatus, the roll-off compensation is different for each magnetic head. Also, it is sometimes desirable to shift the range over which the frequency response is to be compensated to produce a flat response.
While there exist prior art methods which would be suitable for controlling an RF equalizer circuit, these prior art methods cause the control signal to be introduced in the signal being reproduced from the magnetic media. This is obviously undesirable. What is required is a controllable RF equalizer which does not superimpose the equalization control signal upon the output signal.